Ethylene, a light olefin hydrocarbon with two carbon atoms per molecule, is an important building block petrochemical. The primary use for ethylene is as a monomer for the production of polyethylene for both linear low density polyethylene and high density polyethylene. Other uses include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohols. Essentially all of the ethylene is produced by the steam cracking or pyrolysis of hydrocarbons. Hydrocarbons used as feedstock for ethylene plants include natural gas, naphtha, and gas oils. The natural gas components are generally paraffinic and include ethane, propane, and butane. Ethylene is co-produced with propylene and butylenes. Depending upon the feedstock, the products of commercial ethylene plants can include higher olefinic and aromatic hydrocarbons with more than 4 atoms per molecule.
An ethylene plant is a very complex combination of reaction and gas recovery systems. The feedstock is charged to a cracking zone in the presence of steam at effective thermal conditions to produce a pyrolysis reactor effluent gas mixture. The pyrolysis reactor effluent gas mixture is stabilized and separated into purified components through a sequence of cryogenic and conventional fractionation steps. A typical ethylene separation section of an ethylene plant containing both cryogenic and conventional fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti, entitled, "Better Ethylene Separation Unit." The article appeared in HYDROCARBON PROCESSING MAGAZINE, November 1988, pages 57-61 and is hereby incorporated by reference.
Methods are known for increasing the conversion of portions of the products of the ethylene production from a zeolitic cracking process to produce more ethylene and propylene by a disproportionation or metathesis of olefins. Such processes are disclosed in U.S. Pat. Nos. 5,026,935 and 5,026,936 wherein a metathesis reaction step is employed in combination with a catalytic cracking step to produce more ethylene and propylene by the metathesis of C.sub.4 and heavier molecules. The catalytic cracking step employs a zeolitic catalyst to convert a hydrocarbon stream having 4 or more carbon atoms per molecule to produce olefins having fewer carbon atoms per molecule. The hydrocarbon feedstream to the zeolitic catalyst typically contains a mixture of 40 to 95 wt-% paraffins having 4 or more carbon atoms per molecule and 5 to 60 wt-% olefins having 4 or more carbon atoms per molecule. In U.S. Pat. No. 5,043,522, it is disclosed that the preferred catalyst for such a zeolitic cracking process selected from acid zeolites of the ZSM type and borosilicates. Of the ZSM-type catalysts, ZSM-5 was preferred. It was disclosed that other zeolites containing materials which could be used in the cracking process to produce ethylene and propylene included zeolite A, zeolite X, zeolite Y, zeolite ZK-5, zeolite ZK-4. synthetic mordenite, dealuminized mordenite, as well as naturally occurring zeolites including chabazite, faujasite, mordenite, and the like. Zeolites which were ion-exchanged to replace alkali metal present in the zeolite were preferred. Preferred cation exchange cations were hydrogen, ammonium, rare earth metals and mixtures thereof.
European Patent No. 109,059B1 discloses a process for the conversion of a feedstream containing olefins having 4 to 12 carbon atoms per molecule into propylene by contacting the feedstream with a ZSM-5 or a ZSM-11 zeolite having a silica to alumina molar ratio less than or equal to 300 at a temperature from 400 to 600.degree. C. The ZSM-5 or ZSM-1 1 zeolite is exchanged with a hydrogen or an ammonium cation. The reference also discloses that, although the conversion to propylene is enhanced by the recycle of any olefins with less than 4 carbon atoms per molecule, paraffins which do not react tend to build up in the recycle stream. The reference provides an additional oligomerization step wherein the olefins having carbon atoms less than 4 are oligomerized to facilitate the removal of paraffins such as butane and particularly isobutane which is difficult to separate from C.sub.4 olefins by conventional fractionation. In a related European Patent 109060B1, a process is disclosed for the conversion of butenes to propylene. The process comprises contacting butenes with a zeolitic compound selected from the group consisting of silicalites, boralites, chromosilicates and those zeolites ZSM-5 and ZSM-11 in which the mole ratio of silica to alumina is greater than or equal to 350. The conversion is carried out at a temperature from 500 to 600.degree. C. and at a space velocity of from 5 to 200 kg/hr of butenes per kg of pure zeolitic compound. The European Patent 109060B1 discloses that silicalite-1 is an ion-exchanged, impregnated, or co-precipitated form with a modifying element selected from the group consisting of chromium, magnesium, calcium, strontium and barium.
Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483, 4,025,575, 4,252,479 (Chang et al.); 4,496,786 (Santilli et al.); 4,547,616 (Avidan et al.); 4,677,243 (Kaiser); 4,843,183 (Inui); 4,499,314 (Seddon et al.); 4,447,669 (Harmon et al.); 5,095,163 (Barger); 5,191,141 (Barger); 5,126,308 (Barger); 4,973,792 (Lewis); and 4,861,938 (Lewis).
Generally, the heavier olefins having six or more carbon atoms per molecule which are produced in commercial ethylene plants are useful for the production of aromatic hydrocarbons. Portions of the olefin product include olefins with four carbon atoms per molecule. This portion includes both mono-olefins and di-olefins and some paraffins, including butane and iso-butane. Because the portion with four carbon atoms per molecule is generally less valuable and requires significant processing to separate di-olefins from the mono-olefins, processes are sought to improve the utilization of this portion of the ethylene plant product and enhancing the overall yield of ethylene and propylene.
SAPO catalysts are often employed in the conversion of oxygenates into light olefins, particularly light olefins having less than four carbon atoms per molecule. In such processes, the ratio of ethylene to propylene produced on a carbon basis varies from about 0.1 to about ten, and more typically, the ratio of ethylene to propylene ranges from about 0.8 to about 2.5. Methods are sought to alter the product distribution from the oxygenate conversion process for making light olefins to overcome equilibrium limitations of the SAPO catalyst. These and other disadvantages of the prior art are overcome by the present invention, and a new improved process for conversion of oxygenates to hydrocarbons, specifically olefinic hydrocarbons, is provided.
It is an objective of the present invention to provide a commercial process for enhancing the production of ethylene and propylene from the catalytic conversion of butene and heavier olefins.
It is an objective of the present invention to provide a process for increasing the production of ethylene and propylene in a methanol-to-olefin facility by the further conversion of butene and heavier olefins.
It is an objective to provide an economic route for enhancing the production of ethylene and propylene from a catalytic cracking operation in a petroleum refinery.